Denise Fennessey of the Orange County environmental health division says Thursday that field tests indicated that the substance coating two rocks was phosphorous but they'll be sent to a state lab for verification. She also says the flammable chemical could have ignited when exposed to air.

Authorities say the 43-year-old woman's cargo pants caught fire on Saturday and she was hospitalized with third-degree burns to her leg. She remains hospitalized Thursday.

The woman's children had picked up the rocks on San Onofre State Beach in San Diego County, which is near the Camp Pendleton Marine base. A spokesman says the base is working with authorities to determine whether any military material might have contaminated the area.

While the woman was standing in her kitchen Saturday about an hour after returning from the beach, the rocks appeared to spontaneously ignite, burning the woman's right thigh and knee, said Capt. Marc Stone of the Orange County Fire Authority. The woman tried to stop, drop and roll but couldn't extinguish the flames, Stone said.

The rocks are small, smooth, and orange and green in color. They eventually fell from the shorts onto the wooden floor where they continued to burn and fill the house with smoke, Stone said.

The woman's husband managed to get the shorts off his wife and was hosing her down on the front deck with a garden hose when firefighters arrived, Stone said. The woman's husband also suffered second-degree burns to his hand trying to put out the flames.

CBS affiliate CBS2 in Los Angeles reported that her children were unharmed.

Both were taken to Western Medical Center in Santa Ana. Firefighters took the rocks with them to show to doctors, Stone said.

Stone said the paramedic who treated the woman had never seen anything like this in his 27 years responding to calls near the beach. "The rocks were still smoking when firefighters took them to the hospital," Stone said.

Authorities didn't provide any details about what would have caused the rocks to ignite, saying they're undergoing testing to see what happens.